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Space Technology

Satellite Glitch Rekindles GPS Concerns 101

Posted by samzenpus
from the close-enough dept.
coondoggie writes "News today that the Air Force is investigating signal problems with its latest Global Positioning System satellite is likely to rekindle the flames of a congressional report last month that said the current GPS coverage may not be so ubiquitous in the future. The Air Force stated that routine early orbit checkout procedures determined that the signals from the Lockheed-built GPS IIR-2 (M), which was launched in March, were inconsistent with the performance of other GPS IIR-M satellites. The Air Force said it has identified several parameters in the GPS IIR-20 (M)'s navigation message that can be corrected to bring the satellite into compliance with current GPS Performance Standards."
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Satellite Glitch Rekindles GPS Concerns

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  • Re:Soloution? (Score:5, Informative)

    by PeterBrett (780946) on Thursday June 18, 2009 @08:02AM (#28372135) Homepage

    Soloution? Pour more money into NASA!

    Um, the GPS constellation belongs to the USAF.

  • by morgan_greywolf (835522) on Thursday June 18, 2009 @08:27AM (#28372337) Homepage Journal

    Pretty much. Of course, any current problems with GPS are likely to be fixed once GPS III [globalsecurity.org] is up and running. It will combine land-based positioners with satellites that have 500 times the transmitter power. Of course, that 'not so ubiquitous' factor is still there as GPS III will allow the U.S. military to shut down GPS to selected geographic areas at will to all but sanctioned receivers. ;)

  • by CosmicRabbit (1505129) <jppequenao@gmaiDEGASl.com minus painter> on Thursday June 18, 2009 @08:43AM (#28372457)
    Well, from TFA the problem seems to be the new L5 [wikipedia.org] frequency, which is "interfering with other signals from the satellite and reducing their accuracy"
    This is a little more serious than just some glitches in the software. It's a basic design problem.
  • by digitig (1056110) on Thursday June 18, 2009 @09:16AM (#28372827)

    Well, if you don't like it, you're free to build and deploy your own damned GPS system. Don't have hundreds of billions of dollars to spare? Don't have a national interest in precision, real-time mapping? Then shut the hell up.

    You called? [upi.com]

  • by Sporkinum (655143) on Thursday June 18, 2009 @09:18AM (#28372841)

    It's not that it doesn't work, it's just that if your GPS happens to be getting a time signal from that one particular satellite, the accuracy might be degraded. The article said +- 20 ft. That's not a big deal. It may be if you were using the GPS to land a plane though. A GPS only needs 3 time signals to triangulate, but can be more accurate if it can see more signals.

  • by Bakkster (1529253) <`moc.liamg' `ta' `nam.retskkaB'> on Thursday June 18, 2009 @09:19AM (#28372865)

    "... which was launched in March". That is not "getting on a bit" - it says that replacements are not fully-functioning copies of the originals, which is worrying.

    "The Air Force said it has identified several parameters in the GPS IIR-20 (M)'s navigation message that can be corrected to bring the satellite into compliance with current GPS Performance Standards"
    In other words, a small workaround is needed, but the satellite will be just fine. If you only knew how often this happened in engineering.
    "The degraded signals are accurate only to about 20 feet, versus about two feet for typical GPS signals, the article stated."
    Sure, this should have been picked up in testing, but it's not like testing something like this is easy. The accuracy needed to detect something of this magnitude is pretty staggering.

  • Re:Soloution? (Score:3, Informative)

    by RenderSeven (938535) on Thursday June 18, 2009 @10:31AM (#28373809)
    There are also a lot of military R&D programs that are lean, DARPA in particular. They are dubiously famous among contractors for pushing slim margins, compressing schedules, adding features, and generally insisting on maximizing their bang for their buck. Some of their programs may seem silly but many are based on improving military cost-effectiveness and ROI. The latter obviously dont get the same media coverage.
  • Re:Soloution? (Score:3, Informative)

    by TooMuchToDo (882796) on Thursday June 18, 2009 @11:29AM (#28374643)
    Air travel was once only for the rich as well. The wealthy typically underwrite new/cutting edge services/technologies/etc, eventually bringing down their cost.
  • Re:solution: (Score:2, Informative)

    by dimeglio (456244) on Thursday June 18, 2009 @12:14PM (#28375297)

    I'm a man, I know exactly where I'm going and how to get there. If I get lost, I don't need a GPS nor anyone else to get back on track. If I don't get back on track, at least I've discovered new lands and people which is all part of the journey. My moto: delivery in more than 30 minutes or its free.

  • by joggle (594025) on Thursday June 18, 2009 @12:23PM (#28375445) Homepage Journal

    Heck, I work for a GPS company and don't rely on GPS for navigation (we do high-precision applications, not navigation so I don't feel too hypocritical about it). However, this isn't because I don't trust GPS but because I prefer to have a good situational awareness of where I am and where I'm going.

    I have tried using GPS navigation in rental cars for fun but usually their directions aren't as good as Google maps directions and have led me down a dead-end street once late at night.

    The best compromise I've seen so far between maintaining a good idea of what's going on and using GPS is Google maps on the G1. You simply map out your directions before you start driving and then you have the choice of enabling the GPS to see where you are on the route or leaving it disabled if you like. Either way you can still zoom in on the map, check street view, etc without needing to spread out a huge map (or having to print out directions before you take off).

    Note: I wouldn't use this story as a justification for not using GPS. The Air Force likes to maintain a 95% reliability of the constellation remaining fully operational each year (meaning that in the lower 48 that you will get a good position fix virtually all the time). They are worried that in the future they can only estimate an 80% chance of the system staying fully operational. The system would still work even if they don't have 31 satellites working. The minimum number of satellites needed in the constellation to provide good position fixes virtually all the time is 24 (4 good satellites in 6 orbital planes). They have additional satellites up there that are either at their end of life or backup satellites that are ready to take the position of another that becomes disabled. See this PDF if you want all the details: http://www.navcen.uscg.gov/GPS/geninfo/2008SPSPerformanceStandardFINAL.pdf [uscg.gov]

  • by Dare nMc (468959) on Thursday June 18, 2009 @12:45PM (#28375777)

    A GPS only needs 3 time signals to triangulate,

    it can continue to track with 3 time signals (since it can rule out some incorrect solutions, based on a physical movement constraint), but can only triangulate with a minimum of 4 inputs. 3 points works in a 2d (IE draw circles on a paper) sense, GPS are calculating intersections of 3d Orbs. It is also possible for terrestrial GPS to assume your the solution located on the surface of earth. Technically a GPS only needs some 4th bit of info, since you are always moving in relation to the satellites, it is possible to use that movement to fine tune your position over time, but most handheld GPS's don't have that bit of calculus and require at least 4 satellites, then assume a maximum movement if you drop to 3.

  • by JohnnyComeLately (725958) on Thursday June 18, 2009 @01:31PM (#28376533) Homepage Journal
    RichardAtWork nailed most of the response, but the OP never cites an article. With Google, I found an article here [survapedia.com] that explains it's L-5 that's causing the problems. L-5 is being turned on so they have first rights to the new frequency and to meet the deadline associated. GPS still works fine on L1 and L2. I have a feeling there might be some co-site interference that is the issue, although I'd think simple RF issues would have been caught in testing.

    The satellites are not identical by any means. Rockwell built the originals and they were rock solid R&D birds. SVN3 was nearly 13 years old when I (and the crew I was on duty with) turned off the payload back in the early 90s. I used to joke with the Rockwell engineers that if requirements could be stopped, a Block 1 payload/chassis, with Block 2 electrical system would last decades (plural). Instead, we've got L1, L2, M-Code, L-5, NUDET territary sensors, and there's some boxes that only 3 letters now know about... Too much crap on something that's basically just transmitting, "HERE I AM!! (at this time)"

  • by Dare nMc (468959) on Thursday June 18, 2009 @02:51PM (#28378015)

    A false solution way off the earth, but not all of them. If you imagine 2 spheres interacting where the satellites are both straight above you at very close distance, you end up with possible solutions being in one big elliptical arc, a good portion being away from the earth. When you add in a third source, they must intersect in at least 2 locations. But because the satellites are very close together your most likely at 4 intersection points, 2 will be in deep space, 2 will be in the earths sphere. The further apart the centers of the spheres the more circular the solution from 2 satellites, the less likely multiple solutions. So if you got good solutions from satellites on opposite ends of the earth, then 3 is good, you won't get that unless your really high though.

    Of course that is all assuming a perfect measurement, add in a little uncertainty in the time dimension (all of your distances are going to be a slightly different moments in time, but within a few microSeconds) and with the closeness of the GPS satellites (relative to your location) your 4th source is a absolute requirement.

    As you say, a little ground based info goes a long ways, partially because it is from such a different direction and distance, its accuracy can even be less.

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